6th International Building Physics Conference, IBPC 2015The Kamao tumulus which is one of the decorated tumuli in Kumamoto city, Japan does not ordinarily open to the public because of a fear of the deterioration of decorated chamber walls. It seems to be thought that the cause of the deterioration is the dew condensation in the tumulus that is fallen, or flowed down on the decorated wall. The purpose of our study is to propose appropriate measures to prevent dew condensation in performing preservation and an exhibition. In this paper, we investigate the followings: (1) The primary factors of dew condensation (by analizing moist air movement in the stone chamber) (2) The validity of the analysis model of temperature and humidity behavior in the tumulus taken into consideration of the temperature and moisture distribution of the mound and the inside of the tumulus. (3) Whether or not the measure which we shield the entrance door so as not to be exposed to solar radiation is effective to prevent dew condensation. The main results are as follows: (1) The evaporation at the soil surface in the tumulus near the door of the entrance is the main factor of the dew condensation. (2) The calculated results using the analysis model we developed agree with the behavior of measured temperature, humidity and condensation. (3) The counter measure which we shield the entrance door is effective to prevent the dew condensation